In brief: How does the stomach work?

Shape and function

The digestive system is made up of a muscular tube that starts at the mouth and ends at the anus. The stomach is an enlarged pouch-like section of this tube. It is found on the left side of the upper abdomen. The stomach’s shape and size vary slightly from person to person, depending on things like your sex and build, but also on how much you usually eat. It is between 20 and 30 centimeters long on average, and can hold about 1.5 liters of food and drink.

The point where the food pipe leads into the stomach is usually kept shut by muscles in the food pipe and diaphragm. This prevents the contents of the stomach from moving back up into the food pipe. When you swallow, these muscles relax and allow food to enter the stomach.

The upper part of the stomach is called the fundus. It is usually filled with air that enters the stomach when you swallow.

In the largest part of the stomach, called the body, food is churned and broken into smaller pieces, mixed with acidic gastric juice and enzymes, and pre-digested. Towards the lower end, the body of the stomach becomes narrower and joins the first part of the small intestine (the duodenum). The lower opening of the stomach is closed off by a circular muscle called the pyloric sphincter. When this muscle relaxes, the opening widens and allows the pre-digested food to pass into the small intestine bit by bit.

Structure of the stomach

Churning machine made of muscle

The lining of the stomach consists of several layers: the mucous membrane tissue covering the inside of the stomach, and the connective tissue with blood vessels, nerves, and muscle tissue. The muscle layer is made up of three sub-layers that each pull the stomach in different directions. They move the contents of the stomach around so vigorously that solid parts of the food are crushed, churned and mixed into a food pulp.

The muscles also move the pre-digested food towards the small intestine once it is ready. The food stays in the stomach for about three hours before it moves on.

Depending on what you eat, it may stay there for a shorter or longer time. The mucous membrane lining has folds in it that act as “pathways” for liquids to flow through. If the stomach is empty, liquids often enter and leave the stomach within fifteen minutes.

Gastric juice: Acid, salts and enzymes

There are lots of tiny glands in the lining of the stomach. These glands produce digestive enzymes and hydrochloric acid, which, together with other substances important for absorbing nutrients, make up the gastric juice. The body makes about three to four liters of gastric juice each day. The hydrochloric acid breaks down the food, and the digestive enzymes split up the proteins in the food. The acid also kills bacteria.

Some of the glands produce a thick, protective layer of mucus that prevents the acid from attacking the wall of the stomach. This mucus also contains a lot of bicarbonate, which is alkaline (the opposite of acidic) and neutralizes the acid before it reaches the wall of the stomach.

Sources

• Brandes R, Lang F, Schmidt R. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.

• Menche N. Biologie Anatomie Physiologie. Munich: Urban und Fischer; 2023.

• Pschyrembel Online. 2023.

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